The use of foam for dust suppression, and for removing particulate matter from a gas stream, is disclosed in my patent application referred to above. The term "foam", as used herein, designates a mixture of liquid, gas, and a surfactant that gives the liquid a film strength which permits the formation of long lasting bubbles when the mixture is agitated to convert it into a mass of bubbles. The liquid used is normally water, and the gas is usually air, because these ingredients are of low cost; but other gas and/or liquid can be used when compatible with the surfactant.
Various compounds are used as surfactants and these can be purchased on the open market. Some of these compounds are proprietary. The strength of the film depends upon the characteristics of the surfactant, and the amount of the surfactant in the liquid-gas mixture as will be more fully explained.
I have discovered that the reason that ordinary fire-fighting foam does not effectively remove all particulate matter from air, stack gases or other dust-laden gases is because the bubbles of the foam are too large. In this specification, reference to removing particulate matter from gas is to be understood as including the removal of substantially all, and especially small particles, such as dust, from an air stream; all detectable particulate matter carried by stack gases from a furnace; and substantially all small particles of solid matter carried by any stream of gas. Reference to bubbles is to be understood as bubbles of a stable foam, that is, a foam that will last for periods of time, if left standing, such as some fire-fighting foams which last for periods of one-half to three hours.
In order to have small particulate matter efficiently trapped, it is necessary for the particle to contact with a bubble of the foam and burst the bubble. As the bubble bursts, the gas in the bubble escapes; the bubble implodes; and the liquid film of which the bubble was made coats the particle. Small particles do not burst large bubbles and are not wetted or trapped by the foam. The smaller the bubble, the smaller the particles that can be trapped. This invention traps all detectable particles as a result of smaller bubbles made with higher strength foam.
It may seem paradoxical that small bubbles are necessary in order to trap small particles by having the particles burst and implode the bubble, and yet require that the material for making the small bubble foam must have higher tensile strength than the larger bubble foam. My theory on this is that because small bubbles have smaller radii of curvature than larger bubbles, higher film strength is necessary in order to be able to hold the film bent to the extremely short radius of a small bubble.
Without high film strength, the small bubbles burst into one another and form larger bubbles where the radius of curvature is larger and the film is under less tension.
As for the small particles being able to burst small bubbles but not large ones, it may be that the film forming the small bubble is already under sufficient tensile stress so that particles of small size and mass will burst the bubble. It may also be that smaller bubbles have thinner films.
Whatever the explanation, I have discovered that the small bubble foam produces unexpected results. For example, this invention was applied to a stack where State Inspectors required that the owner of the plant reduce the particulate matter from his stack gases or close the plant. This invention, with the small bubble foam, removed substantially all detectable particulate matter from the stack gases and brought the particulate matter lower than requirements of the State Inspector. The difference was so remarkable that the State Inspectors required a second test after checking the accuracy of their detection equipment; and the second test showed the same results.
The foam described in this specification has the bubbles burst by contact with small particles of material (dust) and the bursting bubble wets the particle. Particles as small as one micron are readily wetted. As this effect proceeds, the foam is destroyed by contact with the particles. The wetted particles must then be either
1. brought together,
2. made to contact larger particles, or
3. brought into contact with a surface, which may be additional foam.
If the foam is injected into a free-falling aggregate (at a transfer point between belts, for example, or injected into a crusher along with the aggregate), the mechanical motion of the aggregate will provide the required particle to particle contact. When the foam is injected into an aggregate which is all fines (1 to 200 micron) some means must be provided to cause the wetted particles to coalesce. This is readily accomplished by use of a cyclone, as disclosed in my patent application referred to above, or other centrifugal device. A simple cyclone can be made 99.99% efficient by the injection of the small bubble foam of this invention along with the particle laden input to the cyclone.
I have also discovered that as particles burst the bubbles and destroy progressively more of the foam, it is advantageous to keep the supply of foam sufficient so that there is an excess of foam to form a slurry that will drain from the space where the removal of the particles takes place. A slurry consisting of foam will carry more solid material than an ordinary slurry of water and particulate matter.
Another advantage of small bubble foam is that it can be ejected from nozzles at considerable pressure and resulting high velocity. This will be explained more fully in the description of the preferred embodiment.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.